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Uncovering a 260 Pc Wide, 35 Myr Old Filamentary Relic of Star Formation

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Uncovering a 260 Pc Wide, 35 Myr Old Filamentary Relic of Star Formation MNRAS 000,1{13 (2019) Preprint 14 November 2019 Compiled using MNRAS LATEX style file v3.0 Uncovering a 260 pc wide, 35 Myr old filamentary relic of star formation Giacomo Beccari,1? Henri M.J. Boffin,1 and Tereza Jerabkova,1;2;3;4;5;6 1European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Munchen¨ 2Helmholtz Institut fur¨ Strahlen und Kernphysik, Universit¨at Bonn, Nussallee 14{16, 53115 Bonn, Germany 3Astronomical Institute, Charles University in Prague, V Holeˇsoviˇck´ach 2, CZ-180 00 Praha 8, Czech Republic 4 Astronomical Institute, Czech Academy of Sciences, Friˇcova 298, 25165, Ondˇrejov, Czech Republic 5 Instituto de Astrof´ısica de Canarias, E-38205 La Laguna, Tenerife, Spain 6 GRANTECAN, Cuesta de San Jose s/n, 38712 Brena Baja, La Palma, Spain Accepted 2019 November 13. Received 2019 November 13; in original form 2019 July 04 ABSTRACT Several recent studies have shown that the Vela OB2 region hosts a complex constel- lation of sub-populations with ages in the range 10 to 50 Myr. Such populations might represent the best example of the outcome of clustered star formation in Giant Molec- ular clouds (GMC). We use Gaia DR2 data over an area of 40 deg radius around the open cluster Collinder 135 to extend the study of the stellar populations of the Vela OB2 region over an area of several hundreds of parsecs on sky. Detailed clustering al- gorithms combined with the exquisite astrometric quality of the GAIA catalogue allow us to detect a new cluster named BBJ 1 that shows the same age as NGC 2547 (30 to 35 Myr), but located at a distance of 260 pc from it. Deeper investigation of the region via clustering in 5D parameter space and in the colour-magnitude diagram allows us to detect a filamentary structure of stars that bridges the two clusters. Given the extent in space of such structure (260 pc) and the young age (∼35 Myr), we exclude that such population originates by the same mechanism responsible to create tidal streams around older clusters. Even if we miss a complete picture of the 3D motion of the studied stellar structure because of the lack of accurate radial velocity measurements, we propose that such structure represent the detection of a 35 Myr old outcome of a mechanism of filamentary star formation in a GMC. Key words: open clusters and associations: general { stars: formation { stars: pre- main-sequence 1 INTRODUCTION into low mass stars. However, the vast majority of stellar cores are found at intersections of filaments and they seem Since the seminal study by Lada & Lada(2003), it is gen- to exhibit primordial mass segregation (Plunkett et al. 2018; erally understood that the vast majority of stars are formed Pavl´ık et al. 2019). As an example, Joncour et al.(2018) re- in embedded star clusters. In particular, several observa- arXiv:1911.05709v1 [astro-ph.SR] 13 Nov 2019 cently found that nearly half of the entire stellar population tions in the infrared and sub-mm using the satellite Her- in Taurus is concentrated in 20 very dense, tiny and prolate schel and the Atacama Large Millimeter/submillimeter Ar- regions called NESTs (for Nested Elementary STructures). ray (ALMA) observatory(Andr´eet al. 2014; Hacar et al. Moreover the high-mass stars and embedded star clusters 2018) seem to indicate that most stars form in sub-parsec appear to be exclusively linked to places of high mass sur- high-density regions in molecular clouds (MCs). face density and multiple intersections of filaments and fibers Such high mass density sub-parsec regions in MCs are (Schneider et al. 2012; Hennemann et al. 2012; Hacar et al. made of a complex structure of intersecting molecular small- 2018). scale gas fibers and larger-scale filaments (Hacar et al. 2017, 2018). The density along individual star-forming filaments is By zooming out from individual sub-parsec embedded 5 3 high enough (10 particles/cm ) to gravitationally fragment clusters, or NESTs of star formation, we can see that gas and dust filament complexes can have lengths up to 100pc. For example, Großschedl et al.(2018) has recently used the ? E-mail: [email protected] position and astrometric information of the young stellar © 2019 The Authors 2 Beccari, Boffin & Jerabkova objects (younger than 3Myr) in Orion A and available from stars on the celestial map in right ascension R.A. and dec- Gaia-DR2 to study the 3D shape and orientation of the giant lination Dec. As expected, we could recognize many aggre- molecular cloud (GMC) Orion A. They find that the true gates and clumps of stars likely associated with well know extent of Orion A is 90 pc, more than twice bigger with star clusters. We used the catalogue of open clusters' mem- respect to the projected 40 pc. bers recently published by Cantat-Gaudin et al.(2018) to Moreover the sub-structure of molecular clouds seem identify the position of the know clusters in 3D space in the to be such that filaments & fibers can create large filament surveyed area. Surprisingly, a clump of stars roughly cen- complexes, usually called bones, which can be linked with tered at RA=06:21:08 Dec=-16:10:00 (l=224.221369, b=- galaxy-wide instabilities induced by bar or spiral arms (e.g. 13.866467) escaped the selection done by Cantat-Gaudin Li et al. 2016; Mattern et al. 2018). In the era of Gaia the et al.(2018). No star clusters are found in the close vicinity following question thus emerges: what imprint does this star in the recent work from Bica et al.(2019). The search for formation in seemingly large scale filaments leave in young star cluster-like objects in a 1◦ radius around the position of stellar populations just emerging from their natal molecular the cluster using Simbad (Wenger et al. 2000) gives negative clouds? result. Moreover no cluster appears at this position in the Wide field optical to near-infrared surveys revealed that WEBDA2 data base. when a star forming region is observed and studied at a scale of hundreds of parsecs it shows that young stars aggregate in multiple sub-clusters (Großschedl et al. 2018; Ksoll et al. 3 A NEW STAR CLUSTER: BBJ 1 2018; Zivkov et al. 2018). Very recently, the unprecedented astrometric precision reached through the data taken by the We used the DBSCAN data clustering algorithm (Ester Gaia satellite (Gaia Collaboration et al. 2016) and avail- et al. 1996) implemented as part of the python scikit-learn 3 able to the community trough the second data release Gaia project (Pedregosa et al. 2011) in order to identify the pres- DR2 (Gaia Collaboration et al. 2018), made it possible to ence of a star cluster at the position indicated above. DB- isolate and study the stellar population in clusters with un- SCAN requires the user to control two input parameters, equaled precision (Zari et al. 2018). namely a search radius (eps) and the minimum number of Along this line, the Vela OB2 association has recently points required to form a cluster (min samples). In short revealed to be one of the best test-bench to study the early the algorithm begins with an arbitrary starting point. Then, stages of clustered star formation (SF) at ages 10 to 35 Myr. the point's eps-neighborhood is retrieved and if it contains a Using a combination of Gaia DR2 astrometric information sufficient number of points, a cluster forms. The DBSCAN and accurate photometry, Beccari et al.(2018) recently stud- algorithm does not require one to specify the number of clus- ied a 10 × 5 deg region around the Wolf-Rayet Star γ2 Vel, ters, and it can find arbitrarily shaped clusters in parameter including the well known clusters Gamma Vel and NGC2547. space (Gao 2014). They discovered an assembly of six clusters. Four of them In order to take full advantage of the astrometric in- formed coevally from the same molecular clouds 10 Myr formation provided by the Gaia DR2 catalogue, we per- ago, while NGC 2547 formed together with a newly discov- formed the clustering algorithm in 5D space, hence running ered cluster 30 to 35 Myr ago. Later Cantat-Gaudin et al. the search simultaneously in α, δ, $, µα and µδ. On top (2019a,b) expanded the study on a wider area (∼ 30×30 deg) of the selection of stars in $ mentioned above, we further within the Gum nebula. They hypothesise that the set of 10 restrict the clustering search to an area in Galactic coordi- Myr clusters including Vela OB2 have originated from a en- nates 222 < l < 225 and −15 < b < −13 and −8 < µα < −3 ergetic event like a supernova explosion, which could be the and 3 < µδ < 8. We run DBSCAN using eps=0.550 and cause of the IRAS Vela Shell. In this paper we extend the min samples=10. analysis of the stellar population to a wider region with the The clustering method allowed us to clearly identify the scope of investigating further any sign of formation history new cluster. The population of stars identified by the DB- of the stars in the region dynamically imprinted in the 10 to SCAN algorithm as bona-fide cluster's members are shown 30 Myr complex stellar population. as black solid points on the left panel of Fig.1. As shown on the central panel of Fig.1 the stars do not only clus- ter in the coordinate space, but in proper motion around µα = −5 and µδ = 5 mas/yr. This fact confirms that we have 2 THE DATA SET identified an assembly of stars clustered in space and dynam- ically correlated.
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